1. Field of the Invention
The invention relates to a supply system for a medium, in particular, for an exhaust-gas purification apparatus for the treatment of exhaust gases of an internal combustion engine.
2. Description of the Related Art
Supply systems of the type discussed here are known. The German laid-open specification DE 10 2004 046 881 A1 has disclosed a supply system for an exhaust-gas purification apparatus for the treatment of exhaust gases of an internal combustion engine, which supply system comprises a storage vessel for the medium, a dosing device for dosing the medium into an exhaust line of an internal combustion engine, and a provision line. The provision line has a feed line which serves for the supply of the medium to the dosing device. The provision line also has a return line via which the medium can be returned from the dosing device into the storage vessel. Such supply systems are used in particular for the reduction of nitrogen oxides in exhaust gases of internal combustion engines. As medium, use is made in this case of a reducing agent which is supplied to the exhaust line upstream of a selective catalytic converter for performing a selective catalytic reduction. Preferably, in the selective catalytic converter, ammonia is converted with nitrogen oxides to form nitrogen and water, wherein it is typically the case that, before this, the ammonia is released by hydrolysis from the reducing agent that is injected in liquid form into the exhaust gas. As reducing agent, use is made in particular of a urea solution, typically an aqueous urea solution. To ensure reliable functioning of the dosing device, it is necessary, during the operation of the supply system, for the dosing device to be cooled because it is in contact with the hot exhaust gas or the hot exhaust line. For this purpose, in known supply systems, in particular in the supply system known from DE 10 2004 046 881 A1, it is provided that, during operation, the dosing device is flowed through by the medium, wherein the medium flows from a feed port of the dosing device to a return port of the dosing device.
In the case of supply systems of this type, there is the risk of the reducing agent, which is liquid under normal conditions, freezing in the presence of low ambient temperatures. This can result in damage, in particular to the dosing device. Numerous measures are known for preventing damage from being caused to the dosing device by freezing medium. In particular, DE 10 2004 046 881 A1 proposes a solution for ensuring firstly the best possible insulation of the supply system with regard to the freezing behavior of the medium and secondly the least possible insulation action for good heat dissipation. In the presence of low ambient temperatures, however, there is also the risk of the medium arranged in the storage vessel freezing. Since it is here that the largest volume of medium is typically present, it takes a particularly long time for the medium in the storage vessel to thaw out. During this time, it may be the case that a circuit for the medium along the provision line, which is necessary in particular for the cooling of the dosing device, comes to a standstill because it is blocked by frozen reducing agent in the storage vessel, in particular in the region of ports of the provision line to the storage vessel. It may therefore be the case that, under such operating conditions, in particular even when the medium in the provision line and in the dosing device has already thawed out, the dosing device is not adequately cooled because no medium can be conveyed through the dosing device. Damage to the dosing device owing to elevated thermal loading is then possible in particular because the reducing agent volume in the tank takes considerably longer to thaw out than the very much smaller volume in the provision line.
What is needed in the art is a supply system which does not exhibit the stated disadvantages.